1. Field of the Invention
The present invention is directed at methods for estimating corrections to reduce the impact which tissue induced wavefront aberration has on the quality of medical ultrasound images. The method hence has applications to all situations were ultrasound imaging is used in medicine, but also other areas where ultrasound imaging is applied.
2. Description of the Related Art
Spatial variations in the acoustic properties (mass density and compressibility) are the basis for ultrasound backscatter imaging of tissue. However, large variations combined with complex tissue structures, degrade the images because of the following effects:
i) Interfaces between materials with large differences in acoustic properties can give rise to multiple reflections of the ultrasound pulse. Such multiple reflections are termed pulse reverberations, and add a tail to the propagating ultrasound pulse. If the multiple reflections have sufficient amplitude, they show up as noise in the ultrasound image.
ii) Variations of the acoustic velocity within the complex tissue structures introduce aberrations of the forward propagating acoustic wavefront. This is referred to as wavefront aberration, or wavefront aberration, and destroys the focusing of the beam mainlobe and increases the beam sidelobes.
The reduced focusing of the beam mainlobe due to wavefront aberration, reduces the spatial resolution of the ultrasound imaging system. The pulse reverberations and the increased beam sidelobe levels caused by wavefront aberration, introduce additive noise in the image, which in turn reduces the ratio between the strongest and the weakest scatterer that can be detected in the neighborhood of each other, defined as the contrast resolution of the image. This noise is termed acoustic noise because it is produced by the transmitted ultrasound pulse. Increasing the power of the transmitted pulse will hence not improve the signal to noise power ratio, contrary to what is found with electronic receiver noise. It is, therefore, a challenge to reduce pulse reverberations and wavefront aberrations, in order to increase the ultrasound image quality in many applications of medical ultrasound imaging.
Previously, in U.S. Pat. No. 6,485,423 B2 of Nov. 26, 2002, and U.S. patent application Ser. No. 10/408,538 now U.S. Pat. No. 6,905,465, descriptions are given of methods to reduce pulse reverberations, and to estimate corrections for wavefront aberration. The estimation is based on the backscattered signal from a point scatterer, or from a uniform distribution of random scatterers that are δ-correlated in the spatial coordinate.
This patent advances these methods by improving estimation of correction filters for wavefront aberration using the backscattered signal from a more general random distribution of scatterers. The method allows reduction of the interference from strong, off-axis scatterers, as well as discrimination between multiple mainlobes of the aberrated transmit-beam.